The technical field of this invention is that of nondestructive materials characterization, particularly damage detection and monitoring using sensors.
In some situations, such as on aircraft, the component material of interest has multiple layers and these layers are joined or clamped together by one or more fasteners. These fasteners can have a variety of geometries, but in many situations are threaded or press-fit into the holes in the material layers. When using these fasteners, problems include inconsistency in the clamp-up, preloading of the fasteners and lack-of-grip tolerance, as well as damage in the form of galling between the nut and bolt or damage at the load bearing surface. These variations in fastener installation and damage eventually resulting from usage can lead to reduced aircraft life and substantial inspection and rework costs. Inspections are commonly performed to evaluate the condition of the material near the fastener, but the variability in fastener properties or even the differences in properties between fasteners and the adjacent structural or joined material can mask the response of a crack or other damage to the material itself. One common nondestructive testing method, call bolt-hole eddy current testing (ET), requires the removal of the fastener (by “drilling out”) to perform the inspection.
Another common inspection and nondestructive characterization technique for the inspection of damage around fasteners, termed conventional eddy-current sensing involves the excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This produces a time-varying magnetic field, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks, without removing the fastener. A particular difficulty with eddy-current sensors is the effect of material discontinuities, such as edges of the material for detecting cracks around fasteners. These edges and fasteners can strongly influence the response of the sensor and potentially mask the response of cracks that commonly form at these material discontinuities.
An example of such an eddy-current technique is in U.S. Pat. No. 5,399,968. In this patent, Sheppard, et al. teaches of eddy-current probes for the inspection of cracks or flaws in multi-layered structures. Circular and rectangular probe designs are disclosed, with one or two drive winding coils and arrays of sensing element coils. The probes also use a ferrite core for creating a magnetic circuit that guides the magnetic flux into the test material. U.S. Pat. No. 6,952,095 teaches of the use of surface mounted sensors being mounted near and around fasteners for the inspection of damage.
Spatially periodic field eddy-current sensors have also been developed for material condition assessment. For example U.S. Pat. Nos. 5,015,951 and 5,453,689 described flexible eddy-current sensors that have been used to measure foil thickness, characterize coatings, and measure porosity, as well as to measure property profiles as a function of depth into a component material. Sensor arrays have also been developed, as described for example in U.S. Pat. Nos. 5,793,206, 6,657,429, and 6,784,662.